The present invention relates to a process for conversion of crude hydrocarbon mixtures comprising tertiary olefins and gum-forming constituents and to the thus stabilized crude hydrocarbon mixture. This invention also relates to a process for conversion of a tertiary olefin which contains one or more additional ethylenically or acetylenically unsaturated bonds.
Ether compounds and especially alkyl tertiary alkyl ethers are valuable additives for gasoline blends. Exemplary of such alkyl tertiary alkyl ethers are methyl-tertiary butyl ether (MTBE), ethyl-tertiary butyl ether (ETBE) and methyl-tertiary amyl ether (TAME). These ether compounds are usually prepared by catalytic etherification of tertiary olefins with an alcohol, for example, etherification of isobutylene with methanol (to MTBE), etherification of isobutylene with ethanol (to ETBE) and etherification of 2-methyl-2-butene or 2-methyl-1-butene with methanol (to TAME). The tertiary olefin starting compounds are generally not used in substantially pure form, but in admixture with various other saturated and unsaturated hydrocarbon compounds having about the same number of carbon atoms. Such mixtures are also referred to as crude hydrocarbon mixtures which may be obtained as coproducts in steam cracking or catalytic cracking of hydrocarbon feedstock or other hydrocarbon conversion processes. Typical crude hydrocarbon mixtures are crude C.sub.4, crude C.sub.5, crude C.sub.5-9 and crude C.sub.6-10 mixtures, which are available in large quantities in the petrochemical and refining industries.
It is known to etherify such crude hydrocarbon mixtures to provide etherified crude hydrocarbon mixtures which contain alkyl tertiary alkyl ether compounds, such as for example MTBE, ETBE and TAME.
Under etherification conditions, gum-forming constituents, for example reactive multiple ethylenically or acetylenically unsaturated hydrocarbons, such as linear and branched diolefins and acetylenic compounds, present in crude hydrocarbon feedstock undergo oligomerization reactions leading to the formation of gums. The gums formed in the liquid phase or on the catalyst tend to adhere to or clog the catalyst and so cause a decrease in activity thereof eventually leading to complete deactivation. On the other hand, when the etherified crude hydrocarbon mixtures are used as or blended into gasoline or fuel, the gums contained therein lead to coke formation and deposits in the combustion chamber of an engine. Further, in case the etherified crude hydrocarbon mixture still contains gum-forming constituents, the etherified crude hydrocarbon mixture is not storage stable as these constituents give additional gum formation during storage and use.
In order to prevent gum formation, either during the etherification process or while stored or used, it has been proposed to first selectively hydrogenate the gum-forming constituents to remove excess unsaturation prior to the etherification reaction.
DE-A-3,813,689 discloses a process for the conversion of branched olefins with alkanols in the presence of hydrogen, by using catalytically active clays containing a hydrogenation active metal. The crude hydrocarbon mixture used in the examples of DE-3,813,689 contains only a minor percentage of a diolefin, i.e., 2.7 weight percent of isoprene. A disadvantage of this process is that the clay catalyst has a tendency to collapse at the elevated temperatures required for regenerating the catalyst. Reuse of the clay catalyst therefore is unsatisfactory.
U. S. Pat. No. 5,084,070 discloses a process to prepare a gum-free fuel containing alkyl tertiary alkyl ethers by etherification and hydrogenation of a crude hydrocarbon mixture containing straight chain and/or branched and/or cyclic saturated and mono-unsaturated hydrocarbons with 5 to 8 carbon atoms and furthermore containing gum-forming constituents and containing one or more tertiary olefins with an alcohol and hydrogen over a macroporous or gelatinous cation exchanger in the H.sup.+ or acidic form, the cation exchanger comprising a hydrogenation metal component selected from the groups 6-10 of the Periodic Table of the Elements in the elemental form. The amount of gum-forming constituents allowed in the crude hydrocarbon mixture should not exceed 5 weight percent. A disadvantage of this process is that the cation exchange resins used in the process degrade at the elevated temperatures at which the hydrogenation metal compounds to be applied onto the cation exchange resin by usual methods in the art, such as by impregnation, are decomposed to the elemental form. Accordingly, other measures are required to load the hydrogenation metal onto the cation exchange resin which lead to an expensive catalyst. A further disadvantage of this process is that the hydrogenation metal loaded cation exchange catalyst, due to its thermal instability cannot be regenerated by the usual heat treatment at elevated temperatures so that other more cumbersome and expensive regeneration methods are required.
It is an object of the present invention to provide a stable process for preparing alkyl tertiary alkyl ethers from crude hydrocarbon mixtures comprising tertiary olefins and gum-forming constituents.
It is a further object of the invention to provide a process for preparing alkyl tertiary alkyl ethers from crude hydrocarbon mixtures comprising tertiary olefins and gum-forming constituents to form a stabilized hydrocarbon mixture with a substantially reduced gum content and with a substantially reduced gum-forming ability.
It is another object of the present invention to provide a stabilized hydrocarbon mixture containing alkyl tertiary alkyl ethers for gasoline or fuel blending purposes.
It is yet another object of the present invention to provide a process for preparing alkyl tertiary alkyl ethers from crude hydrocarbon mixtures comprising tertiary olefins and gum-forming constituents in the presence of an easily regenerable catalyst.
It is furthermore an object of the present invention, in a preferred embodiment, to provide a stable process for preparing alkyl tertiary alkyl ethers from crude hydrocarbon mixtures comprising tertiary olefins and high amounts of gum-forming constituents.
It is yet a further object of the present invention, in a preferred embodiment, to provide a process for preparing alkyl tertiary alkyl ethers from crude hydrocarbon mixtures comprising tertiary olefins and high amounts of gum-forming components, to form a stabilized hydrocarbon mixture with a substantially reduced gum content, a substantially reduced gum-forming ability, and an increased amount of alkyl tertiary alkyl ethers also due to conversion of undesired gum-forming constituents to valuable alkyl tertiary alkyl ethers.
It is still another object of the present invention, in a preferred embodiment, to provide a stabilized hydrocarbon mixture with a substantially reduced gum content, a substantially reduced gum-forming ability, and an increased amount of alkyl tertiary alkyl ethers due to conversion of undesired gum-forming constituents, having added value for gasoline or fuel blending purposes.
It is furthermore an object of the present invention to provide a process for conversion of a tertiary olefin which contains one or more additional ethylenically or acetylenically unsaturated bonds.